1. Field of the Invention
The present invention relates to a thermosensitive recording material on which colored images are formed by heating. More particularly, the present invention relates to a thermosensitive recording material capable of forming thereon colored images having a high persistency for a record storage thereof for a long time.
The thermosensitive record material of the present invention is able to record thereon colored images exhibiting an excellent resistance to moisture, heat, oily and fatty substances, and plasticizers, and thus has a superior stability when stored for a long time, and therefore is useful as colored image-recording sheets, sheets for use in facsimiles, word processors, CRT image printers and cash dispensers, as passenger tickets, commuter's passes, labels such as POS labels, cards such as prepaid cards, and as transit passes.
2. Description of the Related Arts
It is known that a conventional thermosensitive recording material comprises a supporting substrate, for example, a paper sheet, synthetic paper sheet, or plastic resin film, and a thermosensitive colored image-forming layer formed on a surface of the supporting substrate and comprising an electron-donative dye precursor, for example, a leuco basic dye, an electron-acceptive color-developing agent consisting of an organic acid substance, for example, a phenol compound, and a binder. When the thermosensitive colored image-forming layer is heated, colored images are recorded thereon by a reaction of the dye precursor with the color-developing agent.
This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication Nos. 43-4,160 and 45-14,039 and Japanese Unexamined Patent Publication No. 48-27,736, and is widely employed in practice.
Namely, the thermosensitive recording material is advantageous in that colored images can be easily formed only by heating, and the recording apparatus can be made relatively compact and small size, has a low cost, and is easily maintained, and thus is useful as an information-recording material for various outputs or printers used with, for example, computers, facsimile machines, automatic ticket-vending machines, scientific measurement recorders, and CRT medical measurement recorders.
Nevertheless, the conventional dye-forming type thermosensitive recording materials in which the thermosensitive colored image-forming layer comprises a conventional color-developing agent together with the dye precursor and the binder, is disadvantageous in that the reaction of the dye precursor with the color-developing agent is reversible, and thus the resultant colored images fade with a lapse of time. This fading of the colored images is accelerated by exposure to light, high temperatures and high humidity, and promoted by contact with an oily or fatty substance or a plasticizer, and the colored images fade to an extent such that the faded images cannot be recognized.
With the expansion of the scope of application of the thermosensitive recording material, a possibility of contact of the thermosensitive recording materials with cosmetic creams, oils or plastic polymer articles containing plasticizer, or of exposure to hard conditions, for example, high temperature and high humidity, is increased. For example, when colored images formed in a thermosensitive recording material containing a conventional color developing agent comprising 2,2-bis(4-hydroxyphenyl)propane, i.e., bisphenol A, or benzyl p-hydroxybenzoate (disclosed in Japanese Unexamined Patent Publication No. 52-140,483) is exposed to a high temperature and high humidity condition, the color density of the colored images is lowered. Also, when brought into contact with the plasticizer or the oil or fat substance, the colored images are faded and become unreadable.
Many attempts have been made to inhibit the color-fading of the colored images formed on the conventional thermosensitive colored image-forming layer containing a substantially colorless dye precursor comprising a lactone ring compound.
For example, Japanese Unexamined Patent Publication Nos. 60-78,782, 59-167,292, 59-114,096 and 59-93,387 disclose a thermosensitive colored image-forming layer containing a phenolic antioxidant.
Japanese Unexamined Patent Publication No. 56-146,796 discloses a protective layer formed from a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer.
Japanese Unexamined Patent Publication No. 58-199,189 discloses an intermediate layer formed from a water-soluble polymeric compound solution or a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer, and a surface layer formed from a solution of a hydrophobic polymer in a solvent on the intermediate layer.
Japanese Unexamined Patent Publication No. 62-164,579 and No. 60-219,088 discloses a thermosensitive colored image-forming layer containing an additive consisting of an epoxy compound and/or an aziridine compound.
In the thermosensitive colored image-forming layer containing the phenolic antioxidant, the resultant colored images exhibit a higher resistance to heat and moisture and a longer persistency in the ambient atmosphere than those of a conventional colored image-forming layer free from the phenolic antioxidant, but the improvement in the storage stability of the resultant colored images is still not satisfactory. Also, the phenolic antioxidant does not effectively enhance the resistance of the colored images to the oily or fatty substances, for example, salad oil, and plasticizers, for example, dioctyl phthalate. The resistance of the colored images to oily or fatty substance or plasticizer is determined in such a manner that an oily or fatty substance, for example, a salad oil, or a plasticizer, is brought into contact with colored images, the colored images are left in contact with the oily or fatty substance or the plasticizer for a predetermined time, and then a retention of the color density of the tested colored images is measured in comparison with an initial color density thereof.
When the protective layer or the intermediate and surface layers are formed on the thermosensitive colored image-forming layer, the resultant colored images have a satisfactory resistance to the oily and fatty substances and to the plasticizers, and exhibit a significantly enhanced storage persistency when the salad oil or the dioctyl phthalate is brought into contact with the colored image-forming surface of the recording material. Nevertheless, when the salad oil or the dioctyl phthalate is brought into contact with an edge face of the recording material, it penetrates into the inside of the recording material and causes the colored images to be substantially completely faded. Therefore, the provision of the protecting layer or the intermediate and surface layer cannot completely eliminate the undesirable color-fading of the images.
The addition of the epoxy compound and/or aziridinyl compound to the colored image-forming layer is unsatisfactory in that it takes a long time to satisfactorily stabilize the colored images formed on the colored image-forming layer by a heat-recording operation, and therefore, if an oily or fatty substance, for example, salad oil, or a plasticizer, for example, dioctyl phthalate, is brought into contact with the colored image-forming layer immediately after the heat-recording operation, the resultant colored images are faded to a great extent.